Semiconductor memory devices may be classified into volatile memory devices and non-volatile memory devices according to whether stored information is retained when power to the semiconductor memory devices is off. The volatile memory devices such as a dynamic random access memory (DRAM) may be refreshed by sensing stored data and recharging sensed data to memory cells through bit lines. As operational temperature of the semiconductor memory device becomes higher, leakage current in the memory cell increases, and thus data retention time and refresh period may decrease.